The present invention relates to the perfume industry. It concerns, more particularly, novel optically active isomers of two aliphatic alcohols represented by the formula ##STR3## wherein symbol R.sup.3 stands for a hydrogen atom or a methyl radical, i.e. 1-(2',2',6'-trimethyl-1'-cyclohexyl)-3-hexanol and 1-(2',2',3',6'-tetramethyl-1'-cyclohexyl)-3-hexanol.
Both the alcohols of formula (I) above and their racemic stereoisomers are known compounds. For example, U.S. Pat. No. 4,623,750, belonging to the present applicant, discloses the cis- and trans-configuration isomers of 1-(2',2',6'-trimethyl-1'-cyclohexyl)-3-hexanol having the structures ##STR4## According to this prior art reference, the trans isomer, and the mixtures rich in said isomer, present a more pronounced animal and ambery odor character than the cis isomer.
The two prior known compounds are racemic species, i.e. each of them is a mixture of four optically active isomers. There is absolutely no mention of the latter in U.S. Pat. No. 4,623,750, nor of any eventually useful properties thereof.
On the other hand, U.S. Pat. No. 4,626,602, which also belongs to the present applicant, discloses, amongst others, 1-(2',2',3',6'-tetramethyl-1'-cyclohexyl)-3-hexanol and its configuration isomers, as well as their use in perfumery. This patent discloses in particular that the trans-configuration isomers of formula ##STR5## i.e. those possessing the three substituents of the six-membered ring (positions 1', 3' and 6') in equatorial position, are judged to be distinctly superior to their cis-configuration homologues, as regards the olfactive qualities, which are said to be distinct from one configuration isomer to the next. Once again, this document mentions only racemic forms of the stereoisomers, in spite of the fact that these alcohols possess several chiral centers and their racemates are therefore mixtures of a plurality of optically active species. There is no citation in the prior art as to whether these optically active isomers could themselves be useful perfuming ingredients.
One of the aims of the man skilled in the art of perfumery is to find new chemicals which are more performant than prior known compounds from an olfactive point of view, either because their odor qualities are more distinctive and original or because their odor is much stronger or yet, if he is lucky, as a result of both of these. Such a skilled man knows well enough that he cannot rely on structural closeness to predict whether a new chemical will be a more interesting fragrance ingredient or, even, whether it will be fragrant at all. Although he cannot be unaware of the role that the optical isomerism of a compound plays in determining its odor properties or, rather, its odor perception by the perfumer [see, for instance, W. Pickenhagen, in ACS Symposium Series 388, chapter 12, p. 151, ed. ACS, Washington DC (1989)], he often finds no motivation to prepare the optically active isomers, particularly when confronted with racemic compounds which possess a plurality of chiral centers, knowing that his task will be extremely difficult and unobvious, as the synthesis of the corresponding pure optical isomers not only requires particular expertise but is also costly. In addition, he may find no reward, as there is no way of predicting, even in the presence of a good racemate, whether his efforts will lead only to the discovery that none of the pure optically active components of said racemate is in any way superior to the latter, as a fragrance ingredient or, at least, not superior enough to justify its inevitably dearer industrial development.
In spite of such difficulties, the synthesis of novel chiral perfuming ingredients is an ever increasing need in this industry. The reason for this springs in an obvious manner from articles such as that of G. Ohloff in Experientia 42, 271 (1986), wherein the author not only impresses upon its reader the importance of every new discovery of fragrant chemicals towards completing an empirically set list of structural conditions or parameters, amongst which chiral activity, disclosed in this article and believed to influence odor perception, but also acknowledges the transient quality, at the present stage of the art, of such lists of structural parameters which, as he admits, every new compound behaving uncharacteristically may well contradict, leading to replacements or alterations (see, for example, the conclusion on page 277). Yet, despite the necessity to continue preparing optically active species of known racemates, the outcome of such an endeavour is rendered all the more uncertain and unobvious by the fact that a racemate does not generally produce a fragrance effect which corresponds to a sum of the individual odor properties of the different optically active isomers present in the racemic mixture, both qualitatively and quantitatively, and this even when there are only two of such isomers in said mixture. The olfactive properties of the racemate can be, and often are, quite different from those of each optically active component of said racemate and, that is why it is impossible to predict the olfactive behaviour of any one isomer on the basis of the knowledge of the racemate, particularly when, as is the case here, there are several optically active isomers in the racemic mixture. The result of the chemist's research in this domain is therefore unpredictable.
It should be further noted that in the present case, the individual optically active isomers cannot be simply separated from the known racemic mixtures, but require specifically designed and sophisticated syntheses, as described further on.
The present invention is yet another example of this reality and it brings precisely a new and unexpected contribution into this field.